The present invention relates to an apparatus for producing pure or ultrapure water. More particularly, the present invention relates to an apparatus for the condensate polishing system in thermal and nuclear power plants.
Mixed beds of strongly acidic cation-exchange resins (hereinafter referred to as SAR) having the greater specific gravity and strongly basic anion-exchange resins (hereinafter abbreviated as SBR) having the smaller specific gravity, are indispensable to apparatus for producing pure and ultrapure water. The standards for the quality of water at the outlet of the mixed bed of resins are the strictest in nuclear power plants employing pressurized water reactors (PWR) and the outlet water is required to contain sodium ion (Na.sup.+) and chloride ion (Cl.sup.-) in amounts not exceeding 0.02 ppb and 0.05 ppb, respectively. The lower the concentrations of these ions, the better. The leakage of these ions from the mixed-bed is governed by the proportions of salt-form resins in the mixed bed if the quality of the inlet water of the demineralization tank and the operating conditions for the mixed bed (e.g. service flow rate) are disregarded. Specifically, the higher the proportions of R--Cl (chloride-form anion resin) and R--Na (sodium-form cation resin), the greater the leakage of Na.sup.30 and Cl.sup.-. The major source of these ions is of course the raw water but they are also formed by the following reasons:
R--Na: Separation between SAR and SBR and subsequent transfer of SBR are incomplete and the SAR in the SBR layer contacts regenerant NaOH to form R--Na.
R--Cl: (1) This partly originates from NaCl present as an impurity in the regenerant NaOH; and (2) As in the case of the generation of R--Na, separation and transfer of SBR is incomplete and the residual SBR in the SAR layer contacts regenerant HCl to form R--Cl.
If H.sub.2 SO.sub.4 is used as a regenerant, sulphate-form resins (R--SO.sub.4) will be generated to cause the problem of SO.sub.4.sup.-- leakage.
Various studies have been conducted with a view to reducing the generation of R--Na and a representative approach is described in Japanese Pat. No. 1027750 (Japanese Patent Publication No. 14718/1980). Of the two principal causes of the generation of R--Cl, the first one has become less significant as a result of recent improvements in the quality of the regenerant NaOH. However, no complete solution has been proposed for dealing with the second cause. The current technology does not provide for complete separation and transfer of SBR and the presence of residual SBR in the SAR layer in an approximate amount of 1-2% of the total SBR is inevitable. This means that for each cycle of regeneration, the R--Cl type SBR forms in an amount of 1-2% of the total SBR and as a result of accumulation of this residual SBR, R-Cl in the mixed resin bed amounts to twenty-odd % of the total SBR at equilibrium.
Conventionally, SBR is separated from SAR and transferred by the following typical procedures. FIG. 6 is a schematic diagram of an apparatus that is employed to perform these procedures. Backwashing water 3 is introduced into the resin-packed tank from below at a linear velocity (hereinafter abbreviated as LV) of 8-12 m/hr. After thorough backwashing to achieve separation between the two resin layers, both SAR and SBR are allowed to settle and sluicing water 6 is introduced into the tank from below at an LV of 2.5-4 m/hr so as to fluidize the SBR layer 2. At the same time, compressed air 7 is introduced from the top of the tank, thereby transferring SBR to the anion regeneration tank (not shown). Not only the SBR layer 2 but also SAR layer 1 is slightly fluidized by supplying the sluicing water 6.
In FIG. 6, the open end 4' of a resin transfer pipe 4 is disposed on the central axis of the tank. Alternatively, The open end 4'may be disposed close to the wall of the tank or it way be in the form of a "trough". For the purpose of minimizing the amount of residual SBR, the pipe 4 is disposed in such a way that the open end 4' is positioned slightly below the interface c between the two resin layers at the time when sluicing water 6 is introduced, and this is a commonly employed practice in commercial operations of condensate demineralization.
However, the above procedures for transferring SBR inevitably cause a portion of SBR to remain in the tank in a thickness ranging from a few to twenty-odd millimeters as indicated in FIG. 7 by a hatched area 2'. This can be explained as follows: the farther away from the open end 4' the SBR is situated, the longer it takes for the SBR to reach this open end, with the result that in the mean time the resin situated in the neighborhood of the open end 4' is transferred through the pipe 4; at the same time, the sluicing water 6 being supplied from of the bottom of the tank causes the resin surface to become flat, creating a certain distance between the open end 4' and the resin surface as indicated by l in FIG. 7; as a result, a portion of the SBR remains to be sucked into the pipe 4 through the open end 4' and fails to be transferred through the pipe.
These phenomena do occur even if the flow rate of sluicing water 6 or the ratio of expansion of SAR layer 1 by backwashing is increased or even if the position or geometry of the open end 4' is changed, and complete separation and transfer of SBR cannot be achieved.
Upon closer examination, the SBR that has been left unseparated from SAR after processing in the apparatus shown in FIG. 6 has a profile as depicted in FIG. 8. The residual SBR consists of the following three portions:
X: SBR 2' situated on the surface layer of SAR and which is hatched in FIG. 8 (the same as SBR 2' in FIG. 7);
Y: SBR 2" that is left in the neighborhood of the lower water collecting unit 10 and which is also hatched in FIG. 8; and
Z: SBR left in the interior of the SAR layer.
Portion Y of the residual SBR is unavoidably trapped in the neighborhood of the lower water collecting unit 10 when the mixed resin bed is transferred from a demineralizing tank (not shown) to the cation regenerating tank (which also serves as a separation tank), and residual SBR of this type occurs in a substantial amount irrespective of whether the lower water collecting unit is in the form of a perforated plate or a perforated pipe. This resin cannot be displaced or purged by the simple technique of backwashing and its volume sometimes amounts to as many as 1-3% of the total SBR.
The method currently employed to purge the residual SBR 2" is to supply backwashing water together with air that is simultaneously introduced into the lower water collecting unit in the same manner as air scrubbing is performed by conventional means.
Portion Z of residual SBR can be removed by performing thorough backwashing after the problem with Y has been solved. Under ordinary conditions, the residual SBR and SAR is present in an amount of 0.05% or less. The SBR content can be further lowered by selecting an SAR/SBR combination that affords a great difference in both the specific gravity and the particle size distribution.
For successful separation and transfer of SBR, it is important that the actions taken to deal with the problems associated with Y and Z be thorough enough to collect the greatest part of residual SBR as SBR 2' which is indicated by the hatched area on the surface layer of SAR 1 so that it can easily transferred from the tank.
To this end, the following requirements for system design and operation must be satisfied:
(a) Separation between SAR and SBR and subsequent transfer of SBR are as complete as possible;
(b) After SBR transfer, the level of SAR layer 1 is held constant;
(c) These two requirements are met in spite of slight variations in designed flow rates and in the temperature of backwashing water;
(d) The apparatus employed is simple in construction;
(e) High process reliability is ensured in a consistent manner even if the apparatus is operated unattendedly in a fully automatic way; and
(f) The resins employed are wear-resistant.
The requirement (a) is an obvious condition to be satisfied but (b) is also a particularly important condition. In the process depicted in FIG. 6, sluicing water 6 is introduced into the tank from below, so requirement (b) is difficult to meet if the flow rate or temperature of the water fluctuates. Condensate demineralizing plants are typically composed of 2-10 demineralizing tanks and 2-3 regeneration tanks, and if condition (b) is not met, the balance in the quantities of resins is upset progressively and condition (a) also fails to be satisfied, causing deterioration of the performance of the demineralizing tanks.
The process shown in FIG. 6 does not fully satisfy requirements (a), (b), (c) and (d).
Other processes have been proposed for transferring SBR as a separate layer from SAR. The system disclosed in Unexamined Published Japanese Patent Application (kokai) No. 5669/1973 is shown schematically in FIG. 9. In this system, distance A from the interface d between separated stationary beds to an intermediate sluicing pipe 11 is set at a value between 50 and 300 mm, and distance B from d to a resin transfer pipe 4 is set at a value between 0.5.times.A and 0.6.times.A. After separation into two layers, SAR 1 and SBR 2, sluicing water 6 is introduced through the intermediate sluicing pipe 11 in the upper layer of SAR 1 so as to attain a bed expansion ratio of 80-120% for SBR 2 by backwashing, with SBR 2 being selectively transferred through the resin pipe 4 in the lower layer of SBR 2.
This process intends to transfer SBR, but not SAR, so the temperature and flow rate of the sluicing water 6 must be controlled as in the system shown in FIG. 6. Furthermore, this process also fails to fully satisfy requirement (a).
Japanese Utility Model Publication No. 17229/1985 discloses a system which, instead of a simple resin transfer pipe of the type shown in FIG. 6, employs a pipe with an opening that extends across the tank and which is positioned adjacent and in a face-to-face relationship with the header of a perforated intermediate sluicing pipe.
The open side of this transfer pipe is positioned slightly below the interface between two separated stationary resin beds, with sluicing water being introduced through the intermediate sluicing pipe at an LV of 2.5-4 m/hr while compressed air is introduced into the tank from its top, thereby accomplishing selective transfer of SBR. In this process, not only SBR but also the SAR lying above the intermediate sluicing pipe is transferred.
This method fully satisfies condition (b) but not condition (a), and the equipment it employs is complex in composition.
Japanese Patent Publications Nos. 6339/1981 and 20312/1983, as well as British Pat. No. 1,498,139 propose a process in which a plurality of spray nozzles that are disposed at the interface between two separated resin beds, or which are arranged in two rows, one above the interface and the other below said interface, produce horizontal water jets that separate one resin bed from the other and transfers it to a regeneration tank. However, this method also requires the use of complicated equipment and has yet to be commercialized.
The processes proposed in Unexamined Published Japanese patent application (kokai) Nos. 132653/1985, 34745/1985 (corresponding to U.S. Ser. No. 493,828) and German Pat. No. 2,702,987 fail to fully satisfy conditions (a) to (d).